Future demands for high-end CT (computer tomography) and CV (cardio vascular) imaging regarding the X-ray source are higher power/tube current, shorter response-times regarding the tube current, especially when pulse modulation is desired, and smaller focus spots corresponding to the demands of future detector systems.
One key to reach higher power in smaller focus spots may be given by using a sophisticated electron-optical concept. But of the same importance may be the electron source itself and the starting conditions of the electrons. For a thermionic electron emitter for X-ray tubes it may be essential to heat up a metal surface to get electron emission currents of up to 1-2 A. These electron currents within the tube may be necessary for state-of-the-art medical applications. For today's high-end X-ray tubes, directly or indirectly heated thin flat emitters are usually used.
FIGS. 1a and 1b show examples of conventional directly heated thin flat emitters 101, 201 having a rectangular or circular geometry, respectively. The flat electron emission surface 103, 203 is structured to define an electrical path and to obtain the required high electrical resistance. The thin emitter film is fixed at connection points 105, 205 to terminals 107, 207 through which an external voltage can be applied to the structured emission surface in order to induce a heating current for heating the emission surface to temperatures for thermionic electron emission.
As can be seen in FIG. 2, the electron emitter 101 may be mounted with its terminals 107 to a cathode cup 111. For directly heated electron emitters, insulators 113 are set between the terminals 107 and the cathode cup 111 to obtain an electrical circuit for applying electrical current to the electron emitter. Such insulators are not necessary for indirectly heated emitters that are heated e.g. by electron bombardment or by laser irradiation.
The exact position of the upper cathode cup surface 115 with respect to the emission surface 103 may be essential for a well-defined electron focusing behaviour of the cathode cup. However, the temperature of the electron source including the electron emitter and the cathode cup may influence the distance between the emission surface 103 and the cathode cup surface 115. During a medical investigation with a series of X-ray pulses, the temperatures of the terminals 107, 207 and of the cathode cup 111 may change differently. As a consequence, different thermo-mechanical expansions may occur and cause a change in the relative positions between emission surface 103 and upper cathode cup surface 115.
This is illustrated in FIGS. 3a, 3b. During first pulses, terminals 107 and cathode cup 111 are on temperatures that result in a setup as shown in FIG. 3a. Different positions between the emission surface 103 and the cathode cup surface 115 lead to a bending of the equipotential-lines of the electrical field 117. This bending focuses the beam of electrons 119 which is emitted from the emission surface 103. At the end of a series of X-ray pulses, a different temperature distribution may be established. In FIG. 3b the resulting final positions in case of the terminals 107 being on a higher temperature and hence have a larger expansion is shown. The distance between the upper emission surface 103 and the cathode cup surface 115 is reduced. As a result, the electrical field is not bended as strongly as in the former case. Therefore, a different optical behaviour of the entire electron source is given. The focal spot size and shape on the anode may be changed which may lead to a decrease in optical quality, e.g. the spatial resolution.
In other words, the thermal situation may change while doing several serial X-ray pulses. Therefore, the positions of emission surface 103 and cathode cup surface 115 may change which may lead to a different potential characteristics and a different optical situation. The focal spot on the electron beam on the anode may change which may cause a reduction in optical quality of an X-ray photograph.
In DE 10135995 A1, an electron emitter design as shown in FIG. 4 is presented that may reduce this negative influence. A directly heated thermionic flat emitter 301 has a circular emission surface 303 which is subdivided into current paths 304 which are separated by the slits 305 and which are connected to terminals 307. A number of additional segments 309 are connected by respective narrow webs 311 to the outermost interconnects of the emitter but have no connections to one another due to gaps 313.
As can be seen in FIGS. 5a and 5b, the result of the design shown in FIG. 4 may be that a thermal expansion of the terminals 307 shifts the emitting inner emission surface 303 and the colder outer emitter parts with the protruding segments 309 in the same way. I.e., the upper surfaces of both parts are always in-plane. With regard to the electron emission accordingly, this design may geometrically separate the area of bended electrical potential lines 317 and the electron emitting area 303. Accordingly, a change in the bended potential lines may not have a significant influence on the optical properties of the X-ray source anymore.
However, practical use has revealed that also the electron emitter design described in DE 10135995 A1 may have problems concerning the distribution and homogeneity of an emitted electron beam.
There may be a need for an improved thermionic electron emitter and an X-ray source including same providing an improved electron emission characteristics allowing an improved electron emission homogeneity and/or a decreased temperature dependency.